Metal-containing polymers have been used extensively in the past to prepare ceramic objects due to the high ceramic "char" yields which result when such polymers are heated to temperatures approaching 1000.degree. C. Such polymers have thus proven useful for such applications as ceramic powder binders, as precursors to ceramic coatings, as ceramic fiber precursors, and as powder carriers for molding applications. However, despite the high thermal stability of such polymers, and their ability to form ceramic compositions upon thermal decomposition, the mechanical strength of such polymers has limited their utility in room temperature applications.
In contrast, while organic polymers demonstrate marginal high temperature performance, their strength and durability at ambient temperature has resulted in widespread application of organic polymers where metals or wood had previously been used.
Block copolymers have been prepared from a variety of organic polymer systems. U.S. Pat. No. 5,229,468, entitled "Polymer Precursor for Silicon Carbide/Aluminum Nitride Ceramics" which issued in the name of Jensen on Jul. 20, 1993, describes recent work to prepare a novel block copolymer which is a ceramic precursor and which incorporates alternately a multiplicity of units comprising Al--N bonded segments with a multiplicity of units comprising Si--N bonded segments.
Such block copolymers, whether wholly organic in nature or wholly inorganic in nature have been shown to exhibit the desirable characteristics of each of their component compositions.
Recently there has also been some effort in preparing mixed organic/inorganic polymer compositions by the hydrolysis of Si(OR).sub.4 compounds in which R is an unsaturated, polymerizable organic group such as vinyl or allyl, or an acrylate or methacrylate-based group. This work has been motivated by limitations which derive from the insolubility of many important engineering polymers within sol-gel solutions. Free-radical curing of such "sol-gel" processed monomers results in mixed systems demonstrating some of the useful properties of the organic components used in the synthesis of the monomers as well as some of the desirable properties of the inorganic components. Typically, such systems comprise semi-interpenetrating networks composed of linear organic polymers and a three-dimensional SiO.sub.2 network. Representative of such an approach is work described by B. M. Novak and C. Davies in Macromolecules, 991, 24, 5481-5483.
Other work (see, for example, U.S. Pat. No. 4,448,939, entitled "Polyurethanes Prepared Using Poly(Silyldiamines"), which issued in the names of Fasolka et al. on May 15, 1984, is based on the reaction of --Si-- NH--R-- (silyl amine) groups with organic isocyanates. In these compositions polyurethanes comprising the reaction product of an organic polyisocyanate and a poly(silyldiamine) are described. This differs from the present invention wherein metal-nitrogen polymers comprising a multiplicity of metal-nitrogen bonds bonded in sequence serve as the source of electron density for reaction with the multifunctional organic electrophile. Thus, for example, in a preferred embodiment of the present invention, the silicon-containing polymers of the instant invention comprise repeat units in which each nitrogen atom is bonded to two silicon atoms.
Similar work by A. A. Zhdanov et. al. in Polymer Science U.S.S.R., Vol. 23, No. 11, pp 2687-2696 (1981), describes the reaction of a nitrogen-hydrogen bond present in the silyl amine end groups of linear polysilazasiloxanes with carbonate moieties in mixed polycarbonate silazasiloxane compositions. Such silyl amine end groups are formed by the reaction of hydroxyl groups in the organic fraction of the composition with cyclosilazane rings, resulting in ring opening and concurrent formation of the reactive Si--NH.sub.2 moiety. In contrast to the reaction of Zhdanov, the reactivity described in the instant invention comprises the reaction of metal-nitrogen bonds with multifunctional, organic, electrophilic reagents. When the metal-nitrogen polymer comprises, for example, silicon, it is the Si--N bond which reacts with the organic electrophile.
U.S. Pat. Nos. 4,929,704 entitled "Isocyanate- and Isothiocyanate-Modified Polysilazane Ceramic Precursors" which issued in the name of Schwark on May 29, 1990; U.S. Pat. No. 5,001,090 entitled "Silicon Nitride Ceramics from Isocyanate- and Isothiocyanate-Modified Polysilazanes" which issued in the name of Schwark on Mar. 19, 1991; and U.S. Pat. No. 5,021,533 entitled "Crosslinkable Poly(thio)ureasilazane Composition Containing a Free Radical Generator" which issued in the name of Schwark on Jun. 4, 1991, describe the preparation of organic isocyanate-modified silazane polymers by the reaction of less than about 30 weight percent of an organic isocyanate with a polysilazane comprising Si--H bonds, the by-product of reaction being hydrogen gas. Similarly, U.S. Pat. No. 5,032,649 entitled "Organic Amide-Modified Polysilazane Ceramic Precursors" which issued in the name of Schwark on Jul. 16, 1991, and U.S. Pat. No. 5,155,181 entitled "(Thio)amide-Modified Silazane Polymer Composition Containing a Free Radical Generator" which issued in the name of Schwark on Oct. 13, 1992, teach the preparation of organic amide-modified silazane polymers by the reaction of less than 30 wt. % of an organic amide with a polysilazane comprising Si-H bonds, the by-product of reaction being hydrogen gas. These polymers are described as being useful for the preparation of ceramic materials by pyrolysis processing. None of these patents describe a crosslinkable, organic/inorganic "hybrid" polymer comprising a substantial (e.g., &gt;30wt. %) fraction of the organic component, nor the preparation of crosslinkable, organic/inorganic "hybrid" polymers from electrophiles other than isocyanates or amides. Correspondingly, the utility of such polymers in applications not involving pyrolysis to a ceramic material is not described, nor their further reaction with organic electrophiles comprising a multiplicity of electrophilic groups to generate organic/inorganic "hybrid" polymers.
U.S. Pat. No. 3,239,489, entitled "Polyurea-silazanes and Process of Preparation" which issued in the names of Fink et al. on Mar. 8, 1966, describes the preparation of linear as well as crosslinked polymers by the reaction of certain silazanes with di- or polyfunctional isocyanates. By reacting such compositions, both linear and crosslinked polymers can be prepared. However, Fink et al. do not describe the preparation of uncrosslinked reaction products which can subsequently be crosslinked through a defined mechanism which is distinct from the reaction of the silazane with the di- or polyfunctional isocyanate. Such reactivity for isocyanate- and amide-modified polymers such as those described in the preceding paragraph is also not addressed. Furthermore, the preferred liquid reaction products of the instant invention are not described.
To date, no art has described the synthesis of uncrosslinked, but crosslinkable inorganic/organic "hybrid" polymers by the reaction of substantial quantities (e.g., &gt;30 wt. %) of an organic electrophile with a polysilazane, suitable crosslink mechanisms, or the crosslinked compositions obtained therefrom. For other silicon-nitrogen based polymers, as well as aluminum-nitrogen polymers, boron-nitrogen polymers, and block copolymers and terpolymers prepared from, for example, aluminum-nitrogen polymers, boron-nitrogen polymers, and silicon-nitrogen polymers, no systems are known. While the metal-nitrogen polymers suitable for the practice of this invention may comprise any metal, the preferred compositions of this invention comprise metal-nitrogen polymers containing the metals silicon, aluminum and boron. Crosslinking may be effected by, for example, thermal, radiation, ionic, or radical-based mechanisms.
It has been discovered that crosslinked, covalently-bonded block copolymers comprising (1) organic segments derived from organic electrophiles, and (2) inorganic fractions derived from segments of metal-nitrogen polymers, demonstrate the high mechanical strengths of their wholly organic counterparts, as well as the extended high temperature performance of their wholly inorganic counterparts. These characteristics overcome the limitations encountered in wholly organic or wholly inorganic polymers. Some limitations of wholly organic or wholly inorganic polymer have been discussed above. Furthermore, the instant invention provides polymers with properties heretofore unachieveable. The unexpected superior properties of the polymers of the instant invention result from the synergism of organic and inorganic components of the polymers.